Re: [PATCH v6 05/19] mm: memcontrol: decouple reference counting from page accounting

[Roman Gushchin]

On Thu, Jun 18, 2020 at 07:55:35AM -0700, Shakeel Butt wrote:
> Not sure if my email went through, so, re-sending.
> 
> On Mon, Jun 8, 2020 at 4:07 PM Roman Gushchin <guro@xxxxxx> wrote:
> >
> > From: Johannes Weiner <hannes@xxxxxxxxxxx>
> >
> [...]
> > @@ -3003,13 +3004,16 @@ void __memcg_kmem_uncharge_page(struct page *page, int order)
> >   */
> >  void mem_cgroup_split_huge_fixup(struct page *head)
> >  {
> > +       struct mem_cgroup *memcg = head->mem_cgroup;
> >         int i;
> >
> >         if (mem_cgroup_disabled())
> >                 return;
> >
> 
> A memcg NULL check is needed here.

Hm, it seems like the only way how it can be NULL is if mem_cgroup_disabled() is true:

int mem_cgroup_charge(struct page *page, struct mm_struct *mm, gfp_t gfp_mask)
{
	unsigned int nr_pages = hpage_nr_pages(page);
	struct mem_cgroup *memcg = NULL;
	int ret = 0;

	if (mem_cgroup_disabled())
		goto out;

	<...>

	if (!memcg)
		memcg = get_mem_cgroup_from_mm(mm);

	ret = try_charge(memcg, gfp_mask, nr_pages);
	if (ret)
		goto out_put;

	css_get(&memcg->css);
	commit_charge(page, memcg);


Did you hit this issue in reality? The only possible scenario I can imagine
is if the page was allocated before enabling memory cgroups.

Are you about this case?

Otherwise we put root_mem_cgroup there.

Thanks!